High power solid-state lasers, especially ones that utilize solid state gain media and operate at relatively high gain, typically need a mechanism of suppressing the naturally occurring transverse gain that can lead to losses from amplified spontaneous emission (ASE) and/or to parasitic oscillation. Such deleterious ASE and parasitic oscillation effects reduce the gain available to amplify an input pulse in an amplifier application or the resonant mode in a laser application.
One approach that has been utilized to suppress ASE and to suppress the onset of parasitic oscillations involves bonding an absorbing material to the edges of the gain medium (i.e., adding an edge cladding structure). If the index of refraction of the bonded absorbing material substantially matches that of the gain medium, a substantial portion of the ASE is coupled out of the gain media and into the absorbing material before it can reach a level sufficient to depopulate the excited state and thus reduce or clamp the gain. In general, such claddings include a material that is refractive index matched to the laser gain material and which contains a dopant that absorbs at the laser (ASE) frequency. A number of different materials have been used for cladding, ranging from sprayed-on glass fits to liquids to castings of monolithic glass.
As an example, large neodymium glass laser disks for disk amplifiers such as those that were used in the Nova laser program utilized an edge cladding that absorbed at 1 μm. The edge cladding prevented edge reflections from causing parasitic oscillations that would otherwise have depleted the gain. Another approach is to use a room temperature-vulcanized (RTV) silicone rubber that is poured about the peripheral edge of the laser disk. Plates of filter glass are embedded in the rubber to absorb ASE. This approach provides a low-cost edge cladding that can be used on a large laser system that incorporates glass gain media.
In crystal and/or ceramic media, the index of refraction is usually higher than that of glass (e.g., up to about 1.9) and thus an edge cladding material with an index of refraction of about 1.5 cannot effectively couple out ASE. For normal incidence, the fraction of light reflected in propagating from a material of index n1 to a material of n2 is given by
  R  =                    (                                            n              2                        -                          n              1                                                          n              2                        +                          n              1                                      )            2        .  
For light propagating from a material of index 1.9 into a material having an index of 1.5, 1.4% of the light is reflected. For steeper angles, the reflection percentage gets substantially higher and at the critical angle, all of the light is totally internally reflected. One approach to provide an edge cladding is to diffusion bond a doped crystal of the same material to the outer edges of the gain crystal in order to absorb the ASE due to the gain media. However, because diffusion bonding often requires mating two very flat (<10/λ) surfaces and applying both pressure and heat, it is a very difficult, expensive, and time consuming process with low yield and bonds may have gaps or fail in operation. In addition, since the main crystal and edge cladding crystal are in intimate contact after diffusion bonding, heating of the edge cladding by the ASE introduces stresses across the bond interface, which can fracture either the crystal or the edge cladding. Such an approach is also time consuming and expensive.
Another approach to reduce the level of transverse ASE is to roughen the edges of the gain media with bead blasting or other means. This creates very small reflection sites at the edge of the gain media, which generates large diffraction losses at the edges. However, such a technique, on its own, does not in general sufficiently defeat ASE gain. Other approaches including using a dye or liquid containing the absorber and flowing it around the edge of the slab, painting the edge of the slab with a solid state absorber, and the like, are characterized by other drawbacks. Thus, there is a need in the art for improved methods and systems for edge cladding high power gain media to reduce transverse ASE.
In the past, this has been accomplished with Ti:sapphire and other gain media with the following methods: bonding or gluing an absorber material to the edge of the slab. These methods have all been initially successful, but suffer from an inherent risk of failure as the bond or glue fails, the dye or liquid flow stops, the paint is scratched or burned, or the like.